Surgical implant systems have become ubiquitous in modern medicine. Surgical implantable instruments are used to strengthen, stabilize, and align anatomic structures. Spinal implants constitute about 40% of all orthopedic implants. Spinal implant systems are designed to apply and withstand relatively high mechanical forces so as to hold the spine in alignment while fusion takes place.
Pedicle screws are some of the most frequently implanted devices. During a spinal surgery, multiple pedicle screws can be screwed into bone tissue to provide anchor points for a rod having a predetermined shape configured to bring a patient's spine into a proper alignment. Most pedicle screws include two basic categories: rigid screws and polyaxial screws. In rigid screws, the head portion—the component securing the rod—is immobilized with respect to the screw. In polyaxial screws, the head portion can articulate with respect to the screw. Rigid screws have the advantage of being able to be manipulated, but lack the surgical intraoperative flexibility and ease of use that variable angle screws provide. A new category of polyaxial screws has been developed that allows the surgeon to transition a polyaxial screw from a variable angle configuration to a rigid configuration. These screws, however, lack the ability to be surgically manipulated in a sagittal direction.
During spinal deformity surgeries, it is often necessary to de-rotate the vertebral bodies to normalize the spine. Because patient anatomy varies, insertion of fixed angle surgical screws—in which the anchor segment is set at a fixed angle relative to the rod—can be difficult. Polyaxial and multi-axial screws, which allow the screw shank to pivot about the head portion, permit the screw to be tailored to a patient's unique anatomy before and during insertion of the rod.
This problem was first addressed by the device patented in the U.S. Pat. No. 8,277,490 (the '490 Patent), which is incorporated herein by reference in its entirety. The '490 Patent discloses a variable angle surgical screw permitting sequential tightening of the polyaxial rotational components of the head portion and selectively immobilizing the rod within the head portion. The surgical screw disclosed in the '490 Patent allows for correction of any deformity, including rotational, malalignment, longitudinal angulation, compression or distraction, or bust fracture deformities. The head portion described in the '490 Patent allows intraoperative manipulation of the vertebral bodies to create sagittal alignment as desired by the surgeon intraoperatively. This is the first pedicle screw in which the head portion has three-degrees of freedom in both rotation and translational motions.
The invention disclosed herein is a novel and non-obvious improvement of the polyaxial screw head disclosed in the '490 Patent.